Magnitude 3.3 IRISH SEA, UK Sunday, 25 August, 2013 at 09:58:37

Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
A magnitude 3.3 earthquake occurred in the Irish Sea on 25 August 2013. Its epicentre was
approximately 25 km W of Fleetwood, Lancashire. The earthquake occurred at a depth of 5
km (3 miles). An earthquake of this size occurs roughly once every three years in the UK.
The event was preceded by a magnitude 2.5 foreshock about four hours earlier.
The earthquake was felt most strongly in the towns of Fleetwood, Blackpool and Barrow-inFurness.
N
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Where was it felt?
Most shaking was felt within a 30-40 km radius of the epicentre. Maximum recorded intensity of III (slight shaking) in
Blackpool, Fleetwood and Barrow-in-Furness. Weak shaking was felt as far away as Liverpool, the Isle of Man and
Anglesey.
I. Instrumental
Not felt by many people unless in favourable conditions.
II. Weak
Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may
swing.
III. Slight
Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many to do not recognise it as
an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration
estimated.
IV. Moderate
Felt indoors by many people, outdoors by a few people during the day. At night, some awakened.
V. Rather
Strong
Felt outside by most, may not be felt by some people in non-favourable conditions. Dishes and windows may break
and large bells will ring. Vibrations like train passing close to house.
VI. Strong
Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off
shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very
Strong
Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to
moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some
chimneys broken. Noticed by people driving motor cars.
VIII.
Destructive
Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse.
Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture moved.
IX. Violent
General panic; damage considerable in poorly designed structures, well designed frame structures thrown out of
plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Intense
Some well build wooden structures destroyed; most masonry and frame structures destroyed with foundation.
Rails bent.
XI. Extreme
Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII.
Cataclysmic
Total destruction – everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The
ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several
meters. In some cases, even the routes of rivers are changed.
Modified Mercalli Intensity Scale
10 km
Location of shaking reports (image courtesy BGS)
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Earthquake and historic seismicity
The earthquake epicentre (red star) is plotted on the map with regional historic
seismicity.
Historic recorded seismicity of the British Isles
Small earthquakes in this region of the UK are not unusual. The
largest recorded earthquake in this region of the UK was a
magnitude 5.0 event that occurred on 17th March 1843.
Historic
seismicity
in the region
of the
mainshock
(image
courtesy of
BGS)
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Tectonic interpretation
Although the UK does not lie on an active tectonic plate boundary (one of its nearest plate boundaries being
the Mid-Atlantic Ridge – located some 2000 km to the west), stress within the plate is released along preexisting faults within the crust. Some of these ancient faults are generally not exposed at the surface (these are
known as ‘blind faults’) and in areas of low seismicity, such as the British Isles, it is difficult to identify the
causative fault. Furthermore, with such low-magnitude events, it is not possible to determine the earthquake’s
precise mechanism.
Mid-Atlantic
Ridge
Alpine –
Pyrenees
mountain belt
Africa – Eurasia
collision zone
Map of major tectonic
boundaries in Western Europe
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Seismogram recordings from BGS network and locating the earthquake
By finding the difference in arrival times between the P- and S-wave arrivals at different seismic
stations, we can calculate the distance of the earthquake from each receiver (circles). If we do
this for several stations (triangles), we can determine the approximate epicentre of the
earthquake (red star) by finding the common intersection
point of these radii.
P-waves
S-waves
You can plot seismograms like these yourself at www.iris.edu/wilber
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Seismogram recordings from the UK School Seismometer Network
DEOS
(University of
Liverpool)
KEELE
(University of
Keele)
P-waves
S-waves
Magnitude 3.3 IRISH SEA, UK
Sunday, 25 August, 2013 at 09:58:37 UTC
Find out more…
•BGS (British Geological Survey) – seismology and earthquakes – frequently asked questions
http://www.earthquakes.bgs.ac.uk/education/faqs/faq_index.html
•IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes
http://www.iris.edu/hq/programs/education_and_outreach/students
•UK School Seismology Project – classroom activities, videos and support documents
http://www.bgs.ac.uk/schoolseismology/home.html
•USGS (United States Geological Survey) – FAQs, glossary, posters, animations
http://earthquake.usgs.gov/learn/
•EMSC (European Mediterranean Seismological Centre)
http://www.emsc-csem.org/